1. Field of the Invention
This invention relates to the use of pipes for the transportation of fluidized finely divided particulate solids and more particularly to an improvement in such methods whereby the service life of the pipes may be extended.
2. Background Art
In numerous industrial applications such as fluidized catalytic cracking operations, finely divided fluidized particulate solids such as fluid catalytic cracking catalyst particles are transported as a fluidized stream of particulate solids. Such fluidized finely divided particulate solids are extremely damaging to the pipes in which they are transported, particularly at curved surfaces such as elbows and the like. It is not uncommon to find that the life of elbows in pipes used for such applications may be as little as three to four months. Typically, the outer radius of the elbow fails as a result of the gouging erosion resulting from the flow of the fluidized finely divided particulate solids through the pipe and around the elbow. Since such processes are frequently operated and a high degree of reliability is desired, the requirement to shut down the process and replace elbows at this frequency is very disadvantageous. It has been found that fluidized finely divided particulate solids can be transported through such pipes and the life of the curved sections of the pipe, such as elbows, extended by the use of pre-rotation vanes such as Cheng Rotation Vanes marketed by Cheng Fluid Systems, Inc. Cheng Rotation Vanes comprise a set of pre-rotation vanes installed upstream of a pipe elbow to alter the characteristics of the incoming flow to the elbow to compensate for the turning effects of the particular elbow geometry. These pre-rotation vanes have been effective to extend the life of elbows positioned in pipes used for the transportation of fluidized finely divided particulate solids. Such vanes are considered to be known to those skilled in the art and are described in a technical bulletin issued by Cheng Fluid Systems, Inc., Volume 1, No. 2, Sep. 1990.
The pre-rotation vanes are frequently fabricated of carbon steel such as carbon or low alloy steel such as steel containing about 2.25% chrome and about 0.5% molybdenum. Other materials can be used but steel is a frequent material of construction for the pre-rotation vanes. Unfortunately, the steel vanes can be rapidly eroded and degraded by impact with the fluidized finely divided particulate solids so that the pre-rotation vanes themselves begin to be quickly eroded and their effectiveness reduced. To overcome this shortcoming, harder materials have been overlaid on the steel vanes to protect the pre-rotation vanes in such applications. STELLITE (trademark of Stoody Deloro Stellite, Inc. for metal alloys) alloy is one material which has been frequently used as a hard facing overlay. The STELLITE alloy is generally welded onto the carbon steel pre-rotation vanes to produce the final prerotation vanes for inclusion in the pipe. Unfortunately, STELLITE alloy has a Rockwell C hardness of about 70 to 75 which is roughly the same as the Rockwell C hardness of the catalytic cracking catalyst solids. Further, the STELLITE alloy, when welded onto the pre-rotation vanes, results in thicker vanes to the detriment of flow through the pipe. Further, fluidized finely divided fluid catalytic cracking catalyst is frequently transported at temperatures approximating 1300.degree. F. which is outside the upper range generally considered suitable for STELLITE alloy.
Accordingly, an improved method for extending the life of pre-rotation vanes used in pipes and the life of pipes used for the transportation of fluidized catalytic cracking catalyst solids has been sought.